1. Field of the Invention
The present invention relates to a correction method of distortion on an input image.
2. Description of the Related Art
Lately, the improvement of a slip recognition technology using an overhead reader (OHR) has been a major problem in financial optical character reader (OCR) business.
An overhead reader is a stand type image input device using a line or area CCD as a camera element as shown in FIG. 1A. Compared with the conventional contact type image input device, such as an image scanner, etc., an OHR has an advantage that by using an OHR, a user can fill in a slip while inputting an image or can input an image while viewing a table in a slip, which leads to comfortable work.
However, compared with an image obtained by a scanner (hereinafter called a “scanner image”), a slip image obtained by an OHR (hereinafter called an “OHR image”) has degradation in image quality, such as uneven gradation, shadow, image distortion, etc. The main distortion of an OHR image is three-dimension distortion.
A plurality of pieces of three-dimension distortion of an OHR image are classified into the following three categories.                a) Center-folded distortion: Distortion caused on a slip with a fold at the center. There are two types of center-folded distortion: sunken center-folded distortion and raised center-folded distortion, in which the fold is sunk and raised, respectively. FIGS. 1B and 1C show an example of an OHR image with sunken center-folded distortion and an example of an OHR image with raised center-folded distortion, respectively.        b) Circumference distortion: Distortion caused on the circumference of a slip. There are two types of circumference distortion: sunken circumference distortion and raised circumference distortion in which the fold is sunk and raised, respectively. FIGS. 1D and 1E show an example of an OHR image with sunk circumference distortion and an example of an OHR image with raised circumference distortion, respectively.        c) Pamphlet distortion: Distortion caused in the case of a pamphlet. FIG. 1F shows an example of pamphlet distortion.        
In order to use an OHR in financial OCR business, a correction method for overcoming three-dimension distortion in these images must be developed, which is a major problem.
Japanese Patent Laid-open No. 10-150532 discloses a distortion correction method for an OHR image with three-dimension distortion in order to implement high-definition document recognition. This correction method targets one-point stapled distortion caused on a document obtained by stapling several pieces of paper at one point. A plurality of pieces of images photographed from a plurality of top and side viewpoints in order to calculate a three-dimension shape are used to measure three-dimension distortion.
However, since the conventional method for correcting three-dimension distortion uses a plurality of cameras, implementation cost becomes high and wide installation space is required.